change createServer

app.js

-var ding = require('./lib/index');
+var AGIServer = require('./lib/index');
 
 var handler = function (context) {
   context.onEvent('variables')
-  .then(function (vars) {
-    return context.streamFile('beep');
-  });
+    .then(function (vars) {
+      return context.streamFile('beep');
+    });
 };
 
-ding.createServer(handler).listen(3000);
+var agi = new AGIServer(handler);
+agi.start(3000);
+
+//ding.createServer(handler).listen(3000);

lib/index.js

 var Context = require('./context');
 
-var agi = {
-  state: require('./state'),
-  Context: Context,
-  createServer: function (handler) {
-    return require('net').createServer(function (stream) {
-      var context = new Context(stream);
-      handler(context);
-    });
-  }
+var agi = function (handler) {
+  
+  var server;
+
+  var handle = function (stream) {
+    var context = new Context(stream);
+    handler(context);
+  };
+
+  var start = function (port) {
+    server = require('net').createServer(handle).listen(port);
+  }; 
+
+  return {
+    start: start
+  };
 };
 
-module.exports = agi;
+module.exports = agi;
\ No newline at end of file